The long-term objective of the proposed research is to understand the cellular and molecular mechanisms of neuronal plasticity in adult animals. Such plasticity undoubtedly plays a major role in the ability of the nervous system to recover from disease or injury, store information, and learn from experience. Specific experiments will focus on the regulation of nerve and muscle plasticity by androgen hormones. In several species, motoneurons and muscles related to sexual behavior have been shown to be regulated by levels of circulating androgens. The system to be studied consists of the flexor motoneurons and muscles of the forearm in male frogs (Xenopus laevis). These muscles are used to clasp females during mating behavior in such a way as to facilitate external fertilization. The system was chosen for its pronounced sexual dimorphism, wealth of background information, and easy accessibility for morphological, physiological, and behavioral studies. The first specific aim is to investigate the effects of androgens on muscle fibers. Hormone levels will be manipulated by surgical methods. Histology, histochemistry, and contraction measurements will be used to determined whether androgens have specific effects on muscle fiber number of contraction properties. The second specific aim is to test the effects of hormones on synaptic structure neuromuscular junctions. Histology and vital staining with fluorescent dyes will be used to ask whether androgens have effects on pre- or postsynaptic structure, and whether certain types of motor units are preferentially affected. The third specific aim is to test the effects of hormones on synaptic function at the neuromuscular junction. Electrophysiological and light microscopic techniques will be used to measure synaptic safety margin, neurotransmitter release levels, and postsynaptic electrical properties. It is expected that the results of this study will be of great usefulness in guiding future studies of neuronal plasticity in the brain.